The present invention relates to a process for the preparation and isolation of 2-substituted 4-hydroxy-4-methyltetrahydropyranols by reacting 3-methylbut-3-en-1-ol (isoprenol) with the corresponding aldehydes in the presence of a strongly acidic cation exchanger and subsequent isolation or distillative separation in a dividing wall column or in an interconnection of two distillation columns in the form of a thermal coupling. Specifically, the present invention relates to a corresponding process for the preparation and isolation of 2-isobutyl-4-hydroxy-4-methyltetrahydropyran by reacting isoprenol with isovaleraldehyde and subsequent distillative separation in a dividing wall column or in an interconnection of two distillation columns in the form of a thermal coupling.
Tetrahedron Letters No. 51, pages 4507-4508, 1970 describes the reaction of 3-alken-1-ols with aldehydes and their use for producing the aroma chemicals rose oxide and dihydrorose oxide. Also mentioned here is the reaction of 3-methylbutanal with isoprenol under acidic conditions.
Chemistry of Heterocyclic Compounds, pages 1107-1109, 1990 describes the condensation of isoprenol with various aldehydes and ketones to give the corresponding di- and tetrahydropyrans in the presence of silica gel or Al2O3 under solvent-free conditions. Pyranols are obtained here only to a low degree when using Al2O3.
SU 825 528 discloses a process for the preparation of di- and tetrahydropyrans and tetrahydropyranols by reacting 2-methylbuten-1-ol-4 (isoprenol) with aldehydes or ketones in the presence of an acidic catalyst, where the acidic catalyst is used in an amount of from 0.0001 to 0.01% by weight, based on the amount of isoprenol, and the reaction is carried out at a temperature of from 0 to 25° C. in an organic solvent. The catalysts specified are the ion exchange resin KU-2 (sulfonated polystyrene resin), para-toluenesulfonic acid, sulfuric acid, phosphoric acid or perchloric acid. By way of example, the reaction of isoprenol with isobutyraldehyde in the presence of KU-2, inter alia, is described.
EP 1 493 737 A1 discloses a process for the preparation of mixtures of ethylenically unsaturated 4-methyl- and 4-methylenepyrans and the corresponding 4-hydroxypyrans by reacting the corresponding aldehydes with isoprenol, where the reaction is initiated in a reaction system in which the molar ratio of aldehyde to isoprenol is greater than 1, i.e. the aldehyde is used in excess. Moreover, the document discloses the subsequent dehydrogenation of said mixtures to give the desired ethylenically unsaturated pyrans. Suitable catalysts specified for the first reaction step are mineral acids, such as hydrochloric acid or sulfuric acid, but preferably methanesulfonic acid or para-toluenesulfonic acid.
EP 1 516 879 A1 discloses a process for the preparation of ethylenically unsaturated 4-methyl- and 4-methylenepyrans by reacting a corresponding aldehyde with isoprenol under dehydrogenating conditions, where the amount of water in the reactor is up to 0.25% by weight, while the conversion of the starting compound used in deficit is less than 50%. The catalysts specified as being suitable for this are likewise mineral acids, such as hydrochloric acid or sulfuric acid, but preferably methanesulfonic acid or para-toluenesulfonic acid.
JP 2007-154069 relates to 2-substituted 4-hydroxy-4-methyltetrahydropyranols with a content of the cis-diastereomer of from 70 to 95% by weight. Moreover, the document discloses a process for the preparation of same, by reacting isoprenol with a corresponding aldehyde in the presence of an aqueous solution of an acidic catalyst. Here, the reaction has to be carried out at a concentration of the aqueous catalyst solution either in the range from 1 to 10% by weight at a temperature of from 0 to 100° C., or in the region of 10% by weight or above at a temperature of from 0 to 30° C. The possible acidic catalysts mentioned are generally also ion exchange resins.
It is known, for isolating multicomponent systems by distillation, to use e.g. a dividing wall column, i.e. a distillation column with lateral feed positioned between top and bottom and a dividing device in the feed section extending in the longitudinal direction of the column for preventing crossmixing of vapors and/or condensate. However, dividing wall columns have hitherto not been used for separating stereoisomeric alcohols of the formula (I), which usually have a low boiling point difference. The object of the invention is to indicate a process for the preparation and isolation of 2-substituted tetrahydropyranols with which, generally, products with very low boiling point differences can be isolated in the purest form possible from a crude mixture with a low capital and energy requirement.
Distillation columns which comprise a dividing wall are known per se and described e.g. in U.S. Pat. No. 2,271,134, U.S. Pat. No. 4,230,533, EP-A 122 367, EP-A 126 288, EPA 133 510, Chem. Eng. Technol. 10 (1987) 92-98; Chem.-Ing.-Techn. 61 (1989) No. 1, 16-25; Gas Separation and Purification 4 (1990) 109 114; Process Engineering 2 (1993) 33-34; Trans IChemE (1994) Part A 639 644 and Chemical Engineering 7 (1997) 72-76. The dividing wall can be integrated into the column in a fixed manner, e.g. welded in, or else it is attached in the column in a removable manner, e.g. inserted.
The removable fixing offers advantages, such as greater flexibility, simpler packing of the column with internals and low capital costs.
Starting from this prior art, the object of the present invention was to provide a process for the preparation and isolation of 2-substituted 4-hydroxy-4-methyltetrahydropyrans, specifically 2-isobutyl-4-hydroxy-4-methyltetrahydropyran, which makes the desired compounds accessible as far as possible                starting from readily available, inexpensive starting materials,        using readily available, inexpensive reagents        in an operationally advantageous manner,        on an industrial scale,        in a high yield,        in a high diastereomer excess,        with the lowest possible formation of undesired by-products that have to be disposed of,        in extremely high purity and        with as far as possible advantageous olfactory properties.        